1. Field of the Invention
This invention relates to apparatus for coating a stripe of viscous material on a sealing surface, and more particularly is directed to improvements in the application of frit slurry in a closed pattern on the sealing surface of the funnel-shaped portion of a cathode ray tube.
2. Description of the Prior Art
Glass-to-glass seals particularly useful in assembling parts of various types of display tubes, such as, for example, in assembling a glass face plate of a cathode ray tube to a funnel-shaped rear portion, are made by laying down a stripe of frit slurry containing low temperature melting glass particles in a binder in a closed pattern on a sealing surface on one of the two members to be sealed or assembled with each other, placing the other member to be sealed against the coated sealing surface, and heating the assembly to a temperature which causes the frit slurry to fuse and thereby unite the two members to be sealed or joined.
In accordance with the prior art, for example, as disclosed in U.S. Pat. No. 4,262,036, issued Apr. 14 1981, and having a common assignee herewith, the funnelshaped portion of a cathode ray tube is supported with its sealing surface facing upwardly in a horizontal plane, and first and second nozzles having parallel horizontal axes are disposed above the horizontal sealing surface and are moved in opposite directions along a path corresponding to the desired closed pattern on the sealing surface from a first position at which outlets of the first and second nozzles are abuttingly opposed and in mutually blocking relation to a second position at which the first and second nozzle outlets are again mutually opposed. A flow of the frit slurry is supplied under pressure, for example, from a pump or pressurized container, at a constant rate through flexible hoses to the first and second nozzles during the motion of the latter between the first and second positions. The flow of the frit slurry through the flexible hoses to the nozzles is controlled either through the operation of the pump or by a valve provided at an outlet from the pressurized container for the frit slurry. However, such control of the flow of the frit slurry into the flexible hoses leading to the nozzles does not provide adequate control of the flow of frit slurry from the nozzle outlets. More particularly, by reason of the inherent elasticity of the flexible hoses, the commencement of the issuance of frit slurry from the nozzle outlets is somewhat delayed following the opening of the valve at the outlet of the pressurized container or the commencement of operation of the pump, whereas frit slurry continues to flow from the nozzles following the closing of the valve at the pressurized container outlet or the cessation of operation of the pump. Thus, the amounts of the frit slurry deposited from the nozzles adjacent their first and second, or starting and terminating positions, are not precisely controllable and, in any case, are not uniform relative to the amounts of frit slurry deposited along other portions of the closed pattern. In view of the foregoing, it has been proposed in the previously identified U.S. patent to provide a shutter for blocking the outlet of at least one of the first and second nozzles as the nozzles are moved to the second or terminating position notwithstanding the fact that the elasticity of the flexible hose continues to exert a pressure on the frit slurry in back of the closed shutter.
However, in the above described apparatus according to the prior art, when the shutter is opened at the commencement of a coating operation, the pressure on the frit slurry due to the elasticity of the flexible hoses connected to the nozzles interferes with the smooth opening of the shutter and further causes the frit slurry to spurt suddenly from the nozzle outlet or outlets with the result that the frit slurry is initially scattered beyond the sealing edge surface and drops inside and outside the funnel-shaped portion of the cathode ray tube. Further, difficulty is experienced in timing the opening and closing of the shutter in relation to the opening and closing, respectively, of the valve at the outlet of the pressurized frit slurry container, with the result that it is difficult to precisely control the amount of frit slurry applied at the first and second or initial and terminating positions of the nozzles in respect to the closed pattern along which the frit slurry is applied to the sealing surface. By reason of the foregoing difficulties, the thicknesses of the frit slurry at the first and second positions along the closed pattern are not uniform with the amounts deposited at other portions along the closed pattern, and a skilled worker is still required for manually doctoring the applied frit slurry.
Furthermore, since the frit slurry dries readily when it comes in contact with a air, the shutter provided at the nozzle outlet or outlets is susceptible to clogging by the dried frit slurry and much time and expense is required for maintenance of the shutter. The susceptibility to clogging of the shutter further prevents the reliable automatic operation of the apparatus without the need for human supervision.